The present invention relates to a system used detect and remove carbon monoxide from a hydrogen rich stream in a fuel cell to thereby protect and optimize the service life of a fuel cell catalyst.
In the 1960s, fuel cells were used in spacecraft during the Gemini and Apollo programs. Since that time various governments and industry have pumped billions into the development of fuel cells, because of their potential advantages. These advantages include environmental, stealth, efficiency as well as simplicity. In the 1980s General Motors concluded that the proton-exchange membrane (PEM) was well suited to vehicle applications. Fuel cells covert hydrogen and oxygen to water without the need for combustion. The process takes place at a much higher efficiency than heat engines. The theoretical efficiency is about 83% (xe2x80x9cFuel Cellxe2x80x9d Energy Handbook, DOE/IR/05114-1) Oak Ridge; Tenn. June 1982 pages 136 to 144 by S. Glasstone. The PEM operates at about 80 to 90xc2x0 C., which makes its platinum-based catalyst extremely sensitive to carbon monoxide (CO) poisoning.
CO bonds to platinum more aggressively than hydrogen, causing the power density to be greatly reduced. However, pure hydrogen can restore the catalyst. Therefore, it is very important to accurately sense the CO levels within a fuel cell and reduce the CO level during its operation. Similar problems occur in other applications of PEM-type fuel cells as well as other chemical process, which use hydrogen with a catalyst such as ammonia production.
The CO danger associated with gasoline-powered, internal combustion vehicles is well known, as over 1,000 people lose their lives annually due to vehicle generated CO according to the Mayo Clinic. In addition, according to the National Highway and Safety Administration, over 100 people lose their lives annually as a result of CO while the vehicle is moving. In view of these facts, CO sensors and detectors have been developed for gasoline-powered internal combustion engines. Goldstein has a co-pending application describes a means to provide safety to vehicle occupants from CO. In addition, examples of CO sensing means were described by Goldstein and by Goldstein et al, in the form of biomimetic sensors, in U.S. Pat. Nos. 5,063,164 and 5,618,493, which are incorporated herein by reference. The biomimetic sensors disclosed in these patents mimic the human response to CO.
The associated chemistry of palladium, molybdenum and copper were first described by Shuler and Schrauzer, i.e., in U.S. Pat. No. 4,043,934. This mixture was short lived and did not work over a wide range of temperature and humidity conditions as required by current standards. Therefore, advances were made using an organic material with supramolecular properties to stabilize and increase performance. In addition to chemical stability and performance superiority, U.S. Pat. No. 5,063,164 teaches that the presence of a target gas such as CO may be determined by monitoring the sensor with a photon source, i.e., by passing photons of a specific spectral region though the sensor and monitoring the intensity of the photon beam, or by using a pulsed photon source to conserve power with a simple photon detector such as a photodiode. There are a number of other target gas sensors that have been disclosed in the U.S. Pat. Nos. 4,043,934; 5,346,671; 5,405,583; 5,618,493; and 5,302,350, which can detect a target gas such as CO by monitoring the optical properties of the sensor.
Goldstein described several CO detector systems which incorporate this type of optical changing sensors such as the biomimetic sensor as discussed above such as in U.S. Pat. Nos. 5,280,273; 5,793,295, and others such as by Marnie et al that disclose a low cost circuit (Apparatus) with software and method for detecting CO in U.S. Pat. Nos. 5,573,953 and 5,624,848. Goldstein et al further disclosed a digital and rapid regenerating means in co-pending patent applications No. 60/051,038, Ser. No. 80/026,34 and No. 60/076,822 herein incorporated by reference. The gas detector systems include a housing containing photon sources that emit photons in at least a region of the electromagnetic spectrum that the sensor absorbs in response to the CO exposure, and a photodetector sensitive in the corresponding active region of the spectra, a circuit designed to measure the response, and a noise maker or other signal means which are actuated by the circuit and an enclosure. The housing (enclosure) has at least one opening to permit the sound to escape and the CO or other gas to enter. The detector also contains a sensor may be permanent or may be configured with a battery for convenient replacement or may be mounted within the housing designed for easy replacement and with or without a convenient battery replacement means. Several such systems were disclosed in U.S. Pat. No. 5,793,295, which is incorporated herein by reference.
In is, therefore, desired that a CO detection system be constructed for use with a fuel cell to detect and monitor the level of Co within the fuel cell. It is desired that such CO detection system comprise means for additionally controlling the CO level within the cell to protect a CO sensitive catalyst within the cell from CO poisoning. It is further desired that CO detection systems of this invention be capable of providing a signal output that can be used to warn a vehicle driver or occupant that the detected and monitored CO level be above a predetermined level.
There is provided several preferred embodiments of the present invention depending on the application, i.e., an apparatus and method for determining the concentration of any particular CO in a fuel cell hydrogen rich reformer stream. These CO detection systems are used as a control means for a fuel cell reform system including optimizing the reformer operation and secondary and tertiary CO removal systems.
Another important embodiment of this invention is incorporated into both fixed and portable reformer/fuel cell systems, and comprises basic sensor and circuit/software system. A built-in CO safety system can be used to control CO in the reforming process by adjusting various variables including flow, water, air, temperature, pressure and others. If problems with CO occurs, a light on the dash may be actuated as an indicator. For example, if the level of CO increases above a predetermined level, the light can flash or even display the level of hazard by digital or bar graph, or in word by vision or/or voice sounds. Further danger can result in louder warnings which may eventually shut down the fuel system supply to the fuel cell if it is not checked or serviced within a predetermined time period.
One aspect of a biomimetic sensor is that it regenerates in air. Therefore, a multiple sensor system is used with at least one sensor in the air stream and one in the hydrogen stream. A circuit means and microprocessor is used to measure the rate of change and the percent transmission of the sensor. A control means is used to modify the operation of the reformer and associated system to minimize the CO and maximize the efficiency of the fuel cell operation.
One preferred embodiment involves the use of a very low-power technology that also has a long service life, is fail safe, and will operate within a temperature range of from xe2x88x9240xc2x0 C. to 70xc2x0 C. The technology is entitled xe2x80x9cSolid State Infrared Reservoirxe2x80x9d (SIR), and is the subject of a copending application No. 60/051,038 filed Jun. 27, 1998, which uses a sensor that responds to CO by a change in its optical properties for example as described in U.S. Pat. No. 5,063,164, and the improvement patents mentioned below in Example 1, and co-pending applications.
The sensor(s) of this system contain supramolecular complexes coated onto porous transparent elements whose optical properties change in response to CO under a variety of conditions, which can be optimized for specific application and temperatures.
An exemplary apparatus, according to the present invention, comprises a series of LEDs and photodiodes, with the target gas sensors located so that the photons emitted LEDs pass through the sensors and the amount of light transmitted is measured by the photodiode response to that light. Each LED illuminates a corresponding sensor, and the light transmitted through the sensor is received by the photodiode.
For a low-power, low-cost embodiment of the invention designed to determine the concentration of CO as the target gas, it is desirable to change the analog signal to digital (at very low power and low cost), the resulting photocurrent from the photodiode, proportional to the transmitted light received by the photodiode, charges a series of capacitors set to a threshold value programmed into the microprocessor as an action level such as an alarm point, this action level may also be further determine by the CO concentration.
In addition, it is useful to be able to turn on the catalytic converter within a closed environment when CO levels are above a predetermined level, e.g., 9 ppm. The alarm point can also be used to alert the driver about the need to service his vehicle, or alert a maintenance person about the need to provide maintenance to the reformer or components of the fuel cell system such as the hydrogen stream control system.
Several invention embodiments, including both apparatus and methodologies, are described in greater detail below with reference to the accompanying figures. In one preferred embodiment, a microprocessor is used to determine the points and time that the sensor readings are taken, and the alarm or some other action levels, and to store recent data of I(n,t) and dI/dt for the most sensitive sensors, which is in the linear region, and further to determine which equation or look up table to use within the microprocessor system.
One preferred low-cost method of monitoring the optical characteristics of a sensor is to determine the optical change at intervals of time (delta t) or (xcex94t) and place some of this data in a look up table for the case where the target gas is CO as has been described by Marine et al. in U.S. Pat. Nos. 5,573,973 and 5,624,848, incorporated herein by reference.
Preferred embodiments of this invention vary widely depending on the application, e.g., a detector may also be designed to display digitally the levels of CO, which is the subject of a co-pending application.
By this general method of operation, the level of sensed CO is determined by measuring the rate of change of light transmission through the sensor dI/dt and I(n,t) and then depending on that information action is trigger by the circuit.